This research is concerned with the principles controlling the evocation of simple responses and is guided by a form of decision theory in which variability of the decision criterion plays a dynamic role. A major goal is to determine by scaling procedures the functions of time describing the processing of the sensory and associative information produced by stimulation. The criterion is assumed to be influenced by variables generally regarded as controlling motivation, attention, adaptation, and set. Response probability and latency are determined by sensory and associative strength in relation to criterion level and variability. Experimental methods include choice, disjunctive, and simple reaction time, and eyelid conditioning. A number of specific experimental variables are under investigation in this context.